I've admittedly always sucked at casting, but right now I like Gemma Arterton (Quantum of Solace, Prince of Persia) as Gamora.
I'm too lazy to post pics for everyone, and for the most part given masks, cgi etc...the pics don't matter as much. Just had to get the attractive female role out of the way...
For Star-Lord, I like Timothy Olyphant (Justified, Hitman.) He has a great deadpan sense of humor and comedic timing that works for the character, while being able to play a legit bad ass.
Rocket Raccoon is tough. For a full performance, I'd say Peter Dinklage (Game of Thrones) pulling mocap duty and voicing the character. But in terms of just voicing? John Kassir, most famous for voicing the Crypt Keeper in Tales of the Crypt, but also Rocket's own voice in Marvel vs. Capcom 3, works just fine.
Drax falls into that body builder/ big guy pit fall that Thor, He-Man, Superman, and so many comic book characters do. In addition to needing to be incredibly muscular, he should be a little older, given that he has an adult daughter (Moondragon) who may show up in future sequels. I'm still working on him...
Groot...is Groot. Doug Jones maybe. or Andy Serkis. More about Mocap than anything.